Growth and oxidation of Na on Ag(001): a MDS study

Growth and oxidation of Na adlayers on Ag(001) have been investigated by means of metastable de-excitation spectroscopy (MDS) and He reflectivity (R-He) measurements. During Na deposition at 180 K, R-He exhibits maxima at the formation of the c(2 x 2)-Na phase and at the completion of the first layer. Oxidation of the Na film at 180 K by exposure to O-2 goes through the incorporation of dimeric molecular ions and, at larger exposure, of neutral 0, molecules. Successive annealing of the oxidised Na film leads first to the loss of the neutral oxygen molecules at 400 K and finally to the removal of the Na-O film at 750 K. Both Na condensation and oxidation of the Na film induce the lowering of the work function. In both cases (Na deposition and oxidation), the singlet-to-triplet conversion was found to be hindered if the work function is lower than 2.5 eV.